1. Field of the Invention
The present invention relates to a foreign particle inspection apparatus, adapted for detecting a foreign particle deposited on the surface for example of a mask, a reticle or a glass plate (such as glass substrate for liquid crystal display device) to be used in an exposure apparatus for producing a semiconductor device, a liquid crystal display device or the like.
2. Related Background Art
If an exposing mask or reticle (hereinafter collectively called reticle), to be used in the photolithographic manufacture of a semiconductor device or the like, has a surfacial defect such as a foreign particle, there will result a loss in the production yield of the manufactured semiconductor devices. For avoiding such loss, the surface of the reticle has been inspected with a foreign particle inspection apparatus, prior to the exposure operation. Also for avoiding the deposition of such a foreign particle directly on the pattern bearing face or the rear face of the reticle, a dust-preventing film, called a pellicle, may be applied on the reticle. Since such a pellicle is displaced or offset from the pattern bearing face of the reticle, conjugate with the exposed face of the photosensitive substrate (such as wafer), a foreign particle of a given size has less influence when deposited on such pellicle than when directly deposited on the reticle. However, the foreign particle present even on the pellicle affects the result of exposure when it exceeds a certain limit, so the surface of such pellicle and the rear face thereof (facing the reticle) are also inspected for the defect such as a foreign particle, by the foreign particle inspection apparatus. In the following description, the reticle will be explained as the object of inspection, but it will be understood that a reticle bearing a pellicle thereon is also included.
FIG. 16 shows an example of the conventional foreign particle inspection apparatus, wherein a reticle 1 is placed on a stage 2 which is movable in the Y-direction by a drive unit 3. The amount of movement of the stage 2 in the Y-direction is constantly measured by a length measuring device 4 such as a linear encoder, and a position signal S1, indicating the measured value of the length measuring device 4 is supplied to a signal processing unit 5. On the other hand, a light beam L1, emitted from an unillustrated light source (such as a laser light source), is reflected and deflected by a galvano scanning mirror 6 (or a polygon scanning mirror) driven by a drive unit 7, and formed by a scanning lens 8 as a light beam L2 converging onto the reticle 1, thereby effecting a scanning operation in the X-direction, along a scanning line 10 between two points 9A and 9B on the reticle 1. The entire surface of the reticle 1 can be scanned with the light beam, by causing the light beam L2 to effect scanning operation in the X-direction and moving the reticle 1 in the Y-direction by the drive unit 3, with a speed lower than that of said scanning operation.
In case the reticle 1 has a surfacial defect such as a foreign particle 11, scattered light L3 is generated by said foreign particle 11 from the light beam L2. The scattered light L3 is respectively condensed by light-receiving lenses 12.sub.1, 12.sub.2, 12.sub.3 onto light-receiving faces of photodetectors 13.sub.1, 13.sub.2, 13.sub.3 such as photomultipliers, which effect photoelectric conversion on the condensed light to respectively provide detection signals S3.sub.1, S3.sub.2, S3.sub.3 to the signal processing unit 5. The three light-receiving optical systems, respectively containing the light-receiving lenses 12.sub.1 -12.sub.3, are provided at optimum positions for receiving the light, and are designed to independently receive the light from the entire range of the scanning line 10.
The signal processing unit 5, also receiving a deflection angle signal S2 supplied to the drive unit 7 for the galvano scanning mirror 6, can identify the presence of the foreign particle 11 from the detection signals S3.sub.1, S3.sub.2 and S3.sub.3. In this identification, the foreign particle 11 is identified for example only if all the three detection signals S3.sub.1, S3.sub.2, S3.sub.3 become at least equal to a predetermined level, in order not to misunderstand the diffracted light from the proper circuit pattern on the reticle 1 as the scattered light from the foreign particle.
At the same time, based on the position signal S1 from the length measuring device 4 and the deflection angle signal S2 for the drive unit 7 for the galvano scanning mirror 6, corresponding to the detection of the foreign particle in said three detection signals, the signal processing unit 5 can recognize the position of said foreign particle 11. More specifically the X and Y coordinates of the foreign particle 11 can be respectively identified from the deflection angle signal S2 and the position signal S1.
Also, based on a fact that the scattered light L3 becomes more intense as the foreign particle becomes larger, the signal processing unit 5 identifies the size of the foreign particle 11 from the magnitude of the three detection signals S3.sub.1, S3.sub.2, S3.sub.3. Thus the signal processing unit 5 can display, on a CRT display 14, the coordinates (X, Y) of the foreign particle and the size thereof, for example in the form of a table. Otherwise, simultaneous with the scanning of the reticle 1 with the light beam, the coordinates (X, Y) and size of the detected foreign particle can be displayed on the CRT display 14 in the form of a two-dimensional map.
In such conventional foreign particle inspection apparatus, each of plural light-receiving optical systems, provided in different directions with respect to the scanning line 10, independently receives the light from the entire range of the scanning line 10, and the foreign particle is identified by the photoelectric conversion signals obtained from the light received by said light-receiving optical systems. However, with the recent increase in size of the liquid crystal display device or the like, the reticle to be inspected has become larger and it has become difficult to receive, in each light-receiving optical system, the light from the entire range of the scanning line crossing such large-sized reticle.
Also for such longer scanning line crossing the reticle, the signal processing system becomes complex if plural light-receiving optical systems are arranged along such scanning line and the foreign particle is identified by parallel entry of the photoelectric conversion signals obtained from the lights received by the respective light-receiving optical systems. Also the signal processing system becomes larger in proportion to the number of the light-receiving optical systems, so that the inspection apparatus becomes bulkier and requires a larger electric power consumption.